docs: def get_min_max_mag(self):
          """
          :returns: minumum and maximum magnitudes from the underlying MFDs
          """
          m1s, m2s = [], []
          for mfd in self:
              m1, m2 = mfd.get_min_max_mag()
              m1s.append(m1)
              m2s.append(m2)
          return min(m1s), max(m2s)